1. Field of the Invention
The present invention relates to a control of an automatic transmission in a vehicle, and more particularly, to a control of a combination of an engine and an automatic transmission in a vehicle for downshifting the automatic transmission with the vehicle engine continuing to generate substantial power output.
2. Description of the Prior Art
When an automatic transmission in a vehicle is shifted between speed stages according to running conditions of the vehicle such as the running speed of the vehicle and the power output of the engine, the route of transmitting torque in the torque transmission mechanism in the transmission is changed over from one to another thereof available according to selective engagement and disengagement of clutches, brakes and one way clutches incorporated in the transmission. In order to accomplish the changing over of speed stages smoothly with no abrupt change in the output torque of the transmission it is generally desirable that the output torque of the engine is low while the speed stage shifting proceeds. In view of this it has already been proposed to decrease the engine output torque temporarily during the speed stage shifting, particularly in a power-on downshifting in which the transmission is shifted down with the engine being kept on in a substantially power generating condition, as described in Japanese Patent Laid-open Publication 61-18528. In this prior art the recovery of the temporarily decreased engine output torque is controlled depending upon a timer.
On the other hand, it has also been proposed to control friction engaging means such as clutches and brakes by monitoring the output torque of the transmission so that the value of the transmission output torque is brought into coincidence with a target value determined therefor in order to improve the shifting performance of the transmission such as described in SAE Paper 820394. This art is self contained in the control of the automatic transmission.